added test to check natural_abund
Merge remote-tracking branch 'upstream/staging' into staging
Added a abundance_by_Z function to data.pyx
Merge remote-tracking branch 'upstream/staging' into staging
changed abundance_by_Z to module-level dict
made abundance_by_Z interval open-open, added test to test for soundness of abundance values
updated abundance test
Merge branch 'abund' into staging